The UPS runtime

How long a bank actually holds a load up, after inverter losses. The designer below starts from this arrangement; change anything and the numbers follow.

What this arrangement is for

How long a bank actually holds a load up, after inverter losses.

The thing people get wrong

UPS runtime is quoted at the inverter output but the battery pays the input: at 90% efficiency a 300 W load draws 333 W from the pack, and it is the second number that sets the runtime.

Questions

What does 3s2p mean?

Three cells in series, then two of those strings in parallel — six cells in total, at three cells’ voltage and two cells’ capacity. Series multiplies voltage and keeps capacity; parallel multiplies capacity and current capability and keeps voltage. The series number always comes first.

Do I need a BMS?

For any lithium chemistry, yes, and this site will not design a pack without one. A BMS is the only thing that stops one cell being driven under-voltage on discharge or over-voltage on charge, and the divergence between cells is invisible from outside the pack. Lead-acid and NiMH can be built without one; lithium cannot.